Bill -
The Calibrator problem is almost certainly the electrolytic C179 on the
Calibrator board itself. This puts a little hum modulation on the
calibrator signal. It's also possible that C167 has gone up a little in
ESR as well, which might also account for some of the audio hum you are
hearing. Another possible contributor would be C202.
Best idea is a kit from "hayseedhamfest.com", but he is temporarily out
of stock. He has the full kit with ALL electrolytics to drop in.
One other possibility for the audio hum is just the sensitivity of your
'phones'. They're much more sensitive than your speaker and typically
have a lower frequency response as well. The audio stages, like most,
run "wide open" or maximum gain, all the time. The output level is
determined by how much signal is put INTO the AF stages via the volume
control. As a result, the s/n ratio is not very good at the very low
levels, where you have minimum signal and maximum noise, (always maximum
noise/gain,) as opposed to "some" signal with the same "noise" at the
higher levels. It's best to have the AF GAIN control at 10-11 o'clock,
which is normal for the speaker.
One way around this is to put a series resistor either in the R-4C or in
the headphone plug to drop the sensitivity of the phones to something
similar to the speaker. This way you can run AF GAIN up where the s/n
is better without blowing out your eardrums! I'd start with a couple
hundred ohms or so and select whatever lets you set that gain control up
into the "good" region.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Bill Jackson wrote:
I have an interesting problem to present to the group.
I'm in the process of restoring a later model R-4C that has a peculiar audio
problem. The receiver is working fine with excellent sensitivity on all bands,
except that on 80m when I turn on the crystal calibrator, I hear what appear to
be weak, but very raspy spurious birdies in between the 25 KHz markers. When I
vary the volume control, the birdies sweep through the passband of the
receiver. Also, when listening to the audio output with a pair of headphones
when the receiver is muted, I hear the usual low level hum that is typical for
the R-4C, but it changes characteristics over a relatively small range around
the 9 o'clock position of the volume control. The hum does not change in
volume, it just sounds rather raspy over this small range, similar in
characteristic to the birdies I hear in between the 25 KHz markers. Is it
possible the audio output stage is self oscillating at some odd frequency and
is somehow mixing with or modulating the output of the crystal calibrator?
I don't have access to a scope, but I do have access to a good quality Fluke
DVM and an IFR service monitor. I would appreciate any suggestions on where to
start looking. This particular receiver was in storage for about 15 years, so
I wouldn't be surprised if there are some electrolytic capacitor issues.
Thanks in advance.
73 de Bill, K9RZ
_______________________________________________
Drakelist mailing list
[email protected]
http://mailman.zerobeat.net/mailman/listinfo/drakelist
